Prevent nvim_buf_get_name frees buf->b_ffname

[tony84727]

nvim_buf_get_name C API doesn't copy memory but return a String with pointer to buf->b_ffname
rust shouldn't free buf->b_ffname

ref:
https://github.com/neovim/neovim/blob/09dffb9db7d16496e55e86f78ab60241533d86f6/src/nvim/api/buffer.c#L1038
https://github.com/neovim/neovim/blob/09dffb9db7d16496e55e86f78ab60241533d86f6/src/nvim/api/private/helpers.c#L403-L408

Reproduce code

use nvim_oxi as oxi;
use oxi::{Dictionary, Function};

#[oxi::module]
fn buffer_name_double_free_sample() -> oxi::Result<Dictionary> {
    Ok(Dictionary::from_iter([(
        "test",
        Function::from_fn(|()| {
            let buffer = oxi::api::get_current_buf();
            let name = buffer.get_name().unwrap();
            oxi::Result::<String>::Ok(String::from(name.to_str().unwrap()))
        }),
    )]))
}

Calling test function returned by the module twice will get a double free error (found on Mac)

[tony84727]

This might be a breaking change. Str might need to implement some traits. Maybe by duplicating some code from string.rs
The main different between Str and String is the former doesn't implement custom Drop to drop ffi memory.